Many questions arise concerning the matter of following a specific “nusach” or version of prayer in various situations.  If one finds himself in a shul of a different nusach than his own, should he adopt the nusach of the congregation when he is davening with them or not?  

The Talmud Yerushalmi Eruvin at the end of the third perek states: . . . “do not change from the custom of your forefathers…” This text is cited by the Mogen Avraham Orach Chaim 68,1 in reference to t’fila meaning not to change one’s own nusach of t’fila, whether Ashkenaz or S’fard. 

Consequently if for example one who uses “nusach Ashkenaz” is davening with a nusach S’fard minyan, he should maintain his own nusach Ashkenaz. However, in the recitation of the “kedushah” he should receite it in “nusach S’fard” and vice versa.  If he is acting as the Sheliach Tsibur, he certainly should daven in the nusach of the congregation rather than his own nusach. 

In regard to the possibility of changing from one nusach to the other on a permanent basis, although the above quoted Yerushalmi restricts this, but many later poskim ruled that if one desires to change from Ashkenaz to S’fard he may do so since they held on the basis of kabbalistic sources that nusach S’fard was on a higher level.  (see Sheilos Utshuvos Maharashdam Orach Chaim, 35, Divrei Chaim, II, Orach Chaim 8, Marsham III, 162.)  See also Yechaveh Daat III, 6.